You cannot claim a spouse tax offset for any period that you or your spouse (during any period they were your spouse) was entitled to family tax benefit (FTB) Part B at a rate based on having full care of a child (full-care rate). This rule applies even if neither you nor your spouse claimed FTB Part B when eligible to do so. To find out if you or your spouse were eligible for FTB Part B, you will need to read Family tax benefit (FTB) - introduction.